1. Field of Invention
This invention relates to a rectifier module. More particularly, this invention relates to a distortion compensation control method available for a three-phase switch-mode rectifier module with a current mono-direction.
2. Description of Related Art
In the controlling of an electric-driven machine or an induction motor, it is an important issue to adjust a motor speed. A conventional electric-driven machine adopts a DC speed regulating technique, and the applications thereof are limited applications due to big volume and high failure rate of hardware.
A variable-frequency drive (VFD) is an electric driving element using the variable-frequency technology and the microelectronic technology to control an electric power transmission element of an AC motor by changing the frequency and amplitude of a motor operation power source.
The VFD is used for changing the AC power supply frequency and the amplitude of the induction motor, so as to change it's a period of motional magnetic field of the induction motor, thereby achieving the purpose of controlling the rotational speed of the induction motor smoothly. The emergence of VFD simplifies the complicated speed-regulating control. The combination of the VFD and the AC induction motor replaces a DC motor to complete most tasks that originally can only be done by using a DC motor, so that the volume of a circuit system can be decreased and the maintenance ratio can be reduced.
Currently, the mid-voltage variable-frequency speed-regulating system is applied widely and has broad prospects in the aspects of such as a large-scale wind generator, a pump, drafting and gearing. The mid-voltage variable-frequency speed-regulating system needs to have the main functions of a safe and fast frequency control within a wide range; a good grid-side power factor; and good input and output current harmonic waves, etc.
Meanwhile, due to the high requirements on the withstand voltage of a switch element in a mid-voltage (referring to a voltage between 1 kV-35 kV, such as 6 kV in a common application) system, the current most-common mid-voltage variable-frequency speed-regulating system mostly use a multilevel cascade scheme. The multistage transformer can transform a high input voltage of the three-phase electrical grid (at the primary side) into a low operation voltage at the secondary side. Each winding at the secondary side is coupled to a single power unit. Each power unit completes the change from rectifier to inversion for a low operation voltage, so as to implement a variable-frequency speed-regulating function. Through the arrangement of the aforementioned multistage transformer, the issue that the power unit cannot withstand high voltage is solved, and the issue about current harmonic waves at the primary side is also solved.
However, the multistage transformer arranged in the aforementioned conventional mid-voltage variable-frequency speed-regulating system is of large volume and high weight, thus leading to high cost and complex design. Thus, it is an important research issue regarding how to use other speed-regulating system structures to omit the arrangement of the transformer while the same performance is achieved.
Currently, the industry has provided a three-phase switch-mode rectifier module. In a practical circuit application, the three-phase switch-mode rectifier module can be a Vienna rectifier module, which is a multilevel rectifier device. Compared with a general three-level pulse-width modulation (PWM) rectifier, the three-phase switch-mode rectifier module also has features of a simple structure, few switch elements, no risk of bridge arm direct pass and high reliability in addition to a good power factor calibration function and DC voltage control capability which are also owned by the general three-level pulse-width modulation (PWM) rectifier. The three-phase switch-mode rectifier module is very suitable for use in a condition requiring small volume, low cost and no energy feedback.
However, it is still an issue desired to be solved that a conventional three-phase switch-mode rectifier module has dead zones. Referring to FIG. 11 and FIG. 12, FIG. 11 illustrates a schematic simplified view of the three-phase switch-mode rectifier module coupled to the three-phase electrical grid. The conventional three-phase switch-mode rectifier module has a grid-side phase voltage us and a rectifier AC-side phase voltage ur. A reactor Ls is connected between the two phase voltages (us and ur). Due to the voltage drop impact on the reactor Ls by the phase current, when the grid-side phase current is and the grid-side phase voltage us have the same phase, the phase relationship is as shown in FIG. 12, and the grid-side phase current definitely lead the rectifier AC-side phase voltage ur by a certain phase angle difference Δθ. Within the range of the phase angle difference Δθ, the rectifier AC-side phase voltage ur cannot be totally controlled by the controller, but mainly depends on the direction of the phase current until the direction of the outputted AC-side phase voltage controlled by the controller is switched to be the same as that of the phase current.
Thus, the zone of the phase angle difference can be considered as the dead zone of the rectifier, and a distortion will happen to the rectifier AC-side phase voltage in this zone, wherein such a distortion will cause the AC-side phase voltage of the conventional three-phase switch-mode rectifier module to have a very large low-order harmonic, thereby affecting the harmonic of the grid-side phase current. Especially for a mid-high voltage or high-power condition, with the influence of component features, the switching frequency is relatively low, and the harmonic influence brought by this dead zone is more serious, thus resulting in the rise of the total harmonic distortion.